ABSTRACT Antigen presenting cells (APCs) play a critical role in the induction of graft-versus-host (GVHD)/ leukemia (GVL) responses after allogeneic bone marrow transplantation (BMT). Therefore agents that regulate the functions of dendritic cells (DCs), the most potent APCs, might have therapeutic potential in GVH processes. Protein lysine deacetylases, traditionally referred to as histone deacetylases (HDAC) play a pivotal role many biological processes. HDAC inhibitors (HDACi) have been developed as anti-tumor agents and they appear to be well tolerated in human clinical trials; but their immuno-modulatory effects have heretofore been largely unrecognized. Data generated by us demonstrate that HDACi regulate experimental acute GVHD partly through induction of the immuno-suppressive enzyme indoleamine 2, 3 dioxygenase (IDO) in host DCs. These exciting pre-clinical experimental data formed the rationale for the development of a proof of concept clinical trial at our institution ((IRB Approval # 2001.0234), which will test the effects of HDAC inhibition in prevention of acute GVHD. However, the precise molecular mechanisms remain unknown. Preliminary data demonstrate that acetylation of non-histone immuno-regulatory protein, STAT-3, in DCs is critical for the DC suppressive effects of HDACi. Data also show that HDAC-1 enzyme interacts with STAT-3 in DCs. In this proposal we will build on our exciting published and unpublished observations from both murine models and cells from healthy human donors. We will test the central hypothesis that acetylation of the non-histone protein, STAT-3, is critical for the negative regulation of DCs and experimental GVHD after treatment with HDACi. The specific aims (SA) are as follows: Specific Aim (SA) 1: To determine the specific HDAC enzyme critical for acetylation of STAT-3 and regulation of DC function by the HDACi. In this specific aim we will test the hypothesis that HDAC enzyme 1 is critical for STAT-3 acetylation and regulation of the innate and allo-stimulatory functions of DCs. Specific Aim (SA) 2: To elucidate the critical role of STAT-3 in regulation of DCs and GVHD by HDACi. In this specific aim we will explore the hypothesis that acetylation of STAT-3 by HDACi is critical for suppression of DCs and experimental GVHD.